The ceramic coating or enameling of a substrate, especially a metal substrate, is well known. In one practice, two ceramic coats are sequentially applied, the first being called a ground coat and the second, a cover coat. For each coat, a different frit composition is normally used. Each frit composition is milled, applied to the substrate, and after drying, each coat must in turn be fired.
In order to eliminate the ground coat application and all that it entails, a so-called direct-on, or one coat-one fire application, of an enamel or ceramic coat can be used. While excellent results are achieved by direct-on application, the problems of protecting the substrate and of securing good adherence of the ceramic coat to the substrate can be increased by this technique.
Since the substrate is usually metallic, one problem of obtaining good adherence of a ceramic coat to a substrate occurs early in the enameling process, namely, during the firing of the frit composition to cause it to fuse and form a smooth coat over the substrate. At this time, the temperatures involved may reach 1700.degree. F or higher, and the surface of the substrate contacted by the frit can be readily and seriously oxidized. Even though a ceramic coat may ultimately form, its adherence is adversely affected by an interfacial oxide layer between it and the substrate which can cause spalling and blistering. Blistering especially appears to result from underlying rust spots.
The problem of oxidizing a substrate during firing of a frit is peculiarly accentuated if the ultimate ceramic coat is intended to be porous. For example, in the case of enameling the interior of a self-cleaning or continuous-cleaning cooking device, such as the exposed interior walls of an oven, it is desirable to have a porous or gas-permeable ceramic coat. In this instance, the ceramic coat may have an oxidation catalyst associated with it and porosity in the coat provides additional surface area for the adsorption of cooking residues or gases and for their catalytic oxidation. Normally, in firing a ceramic coat onto a substrate, a solid, continuous glassy coat forms such that the substrate is subject to oxidation hazards only at the beginning and until the solid, continuous glassy coat forms. However, in making a ceramic coat porous one also subjects the underlying substrate to oxidative attack throughout the firing and cooling cycles, since the substrate is continuously exposed through the pores of the coat.
An additional problem relating to direct-on or one coat-one fire application of a ceramic coat is that such a technique is usually limited to special and expensive grades of steel for the substrate, such as zero carbon steel which may contain up to about 0.003% carbon. If other metals containing carbon in greater amounts are used as substrates for direct-on enameling, such as cold rolled steel or other carbon-rich materials, the resulting ceramic coat usually has poor adherence and is subject to fish scaling. Bubbles, pits or specks almost inevitably develop in the surface of a single ceramic coat applied to a substrate, marring its uniformity and texture.